1. Field of the Invention
This invention relates to a thin film SOI wafer having a buried oxide film (BOX) for forming a high-speed, low power consumption SOI (silicon on insulator) device, and more particularly to a method of producing a SIMOX (separation by implanted oxygen) wafer in which a buried oxide film is formed by implanting oxygen ions into a surface of a wafer and then annealing at a high temperature.
2. Description of Related Art
As a method of producing a thin film SOI wafer, there have hitherto been known a so-called high-dose SIMOX method wherein a dose in the oxygen implantation is high, and a so-called low-dose SIMOX method wherein oxygen ions are implanted at a dose lower by about one significant digit than that of the high-dose SIMOX method and then the annealing is carried out in a high oxygen atmosphere.
As the low-dose SIMOX method is recently developed a so-called MLD (modified low dose) method wherein the formation of BOX at a lower dose is made possible by conducting a final oxygen implantation at a lower dose and about room temperature to form an amorphous layer, which contributes to the mass production of wafers.
The high-dose SIMOX method is a method wherein oxygen ions are implanted under conditions of an acceleration energy: 150 keV, a dose: more than 1.5×1018 cm−2 and a substrate temperature: about 500° C. and thereafter the annealing is carried out at a temperature of higher than 1300° C. in argon (Ar) or nitrogen (N2) atmosphere containing 0.5-2% of oxygen for about 4-8 hours (see K. Izumi et al., Electron lett. (UK) vol. 14, (1978), p.593). However, the high-dose SIMOX method have problems that the implantation time is very long and the throughput is bad and the dislocation density of SOI layer is as very high as 1×105-1×107 cm−2.
The low-dose SIMOX method improves the above problems of the high-dose SIMOX method, and is typically carried out by implanting oxygen ions under conditions of an acceleration energy: more than 150 keV, a dose: 4×1017-1×1018 cm−2 and a substrate temperature: about 400-600° C. and thereafter conducting the annealing at a temperature of higher than 1300° C. in an argon atmosphere containing 30-60% of oxygen, whereby there is attained a significant quality improvement that the buried oxide film (BOX) is thickened by an internal thermal oxidation (which may be abbreviated as “ITOX”) at the annealing step and the dislocation density is reduced and the like (see S. Nakashima et al., Proc. IEEE int. SOI Conf. (1994), p71-72).
Further, the MLD method is developed as an improved version of the low-dose SIMOX method, and is a method wherein after the existing oxygen implantation at a high temperature (400-650° C.), a further oxygen implantation of a dose lower by one significant digit is carried out at room temperature to form an amorphous layer on the surface of the buried oxide film (BOX) (see O. W. Holland et al., Appl. Phys. Lett. (USA) vol. 69 (1996), p574 and U.S. Pat. No. 5,930,643). According to this method, it is possible to conduct the continuous growth of BOX within a wide and low dose range of 1.5×1017-6×1017 cm−2, and also even in the subsequent ITOX process, the internal thermal oxidation may be conducted at a rate higher by 1.5 times than that of the conventional ITOX. As a result, the BOX film is very near to the thermal oxide film, and a significant quality improvement is attained. In the MLD method, it is common that the annealing is carried out in an Ar atmosphere containing 0.5-2% of oxygen for about 5-10 hours after the ITOX step in order to decrease an oxygen content in SOI layer.
All of the above SIMOX methods include a high-temperature annealing step, and particularly it is known that the annealing conditions largely affect the quality of the SIMOX wafer. At the end of the annealing step of SIMOX is usually conducted a heat treatment in an Ar atmosphere containing about 0.1-2 volume % of oxygen at a temperature of not lower than 1300° C. for 5 to 10 hours. In this case, it is known that oxygen diffused into the inside of SOI layer at ITO step is diffused outward by such a heat treatment to sufficiently lower the oxygen content in the SOI layer and also the residual defect, oxygen precipitates and the like in the SOI layer disappear, while oxygen precipitates in BOX grow and are united with each other to disappear the defect in the BOX layer and further a good SOI/BOX interface is formed. However, the good characteristic on the surface roughness of the SOI layer is not necessarily obtained stably by the conventional methods.